In most industrial and automotive engine applications, an engine-driven cooling fan is utilized to blow or draw air across a coolant radiator or heat exchanger. Usually the fan is driven through a belt-drive mechanism connected to the engine crankshaft.
A typical cooling fan has a plurality of blades mounted to a central hub or hub plate. The hub provides a rotary connection to the belt drive mechanism, for example. The size and number of the fan blades is determined by the cooling requirements for the particular application. For instance, a small automotive fan may only require four blades and have a diameter of less than 300 mm. In larger applications, such as heavy-duty automotive applications, particularly trucks and buses, nine blades or more can be utilized in the fan design and the fan can have an outer diameter of 600 mm or more.
In addition to the number of blades and diameter of the fan, the cooling capacity of a particular fan is also governed by the air flow volume that can be generated by the fan at its operating speed. The air flow volume is dependent upon the particular blade geometry, such as the blade area and curvature or profile, and the rotational speed of the fan. As the cooling fan dimensions and air flow capacity increase, the loads experienced by the fan, and particularly the blades, also increase. In addition, higher rotational speeds and increased air flow through the fan can lead to twisting of the blade and increased noise levels.
In order to address these problems to some degree, certain cooling fan designs incorporate a ring around the circumference of the fan. Specifically, the blade tips are attached to a 360° ring. The ring provides stability to the blade tips and also helps reduce vortex shedding at the blade tips, particularly when the ring is combined with a shroud. The ring also provides increased strength to the fan design and improves the vibration characteristics.
Ring fan designs, therefore, eliminate some of the structural difficulties encountered with unsupported cooling fan configurations. However, in the automotive and industrial cooling environment today wherein the fans need to have less weight and yet provide increased performance characteristics, the operating conditions for these fans has been increased to again push the envelope of the ring fan's capability.
One of the problems with ring-type fans is that in today's environment many fans are molded in one-piece and made of a plastic material. The injection molding process inherently produces weak points in the fan ring caused by plastic knit lines. Also, the centrifugal force exerted on the blade-ring interface caused by the mass inertia of the complete circumferential ring at increased fan speeds, can cause failure of molded fans at that interface.
Consequently, a need has developed for ways to improve the cooling air flow capacity of fans, particularly molded ring-type fans, while at the same time increasing their strength and preventing possible failures. This need becomes particularly acute for large industrial and automotive engines, where the fans are larger and have more mass, and as the operational rotational speeds of the fans increase to meet the increasing cooling demands.